Titan holds clue to faint young sun paradox

EARTH should have been frozen in its youth, when the sun was cooler. Yet the young Earth was warm enough for liquid water. This paradox has puzzled us for decades, but now a look at one of Saturn's moons suggests a new explanation.

For the first 2 billion years of Earth's existence, the sun was up to 25 per cent dimmer than it is today. Our planet's average surface temperature should have been around -10 °C, but the geological evidence is that Earth had liquid water at this time.

Titan, too, is warmer than it should be. This is because its atmosphere is so rich in hydrogen and nitrogen that the molecules keep colliding under pressure, leading to a chemical reaction that traps the sun's energy.

To see whether something similar could have happened here, Robin Wordsworth and Raymond Pierrehumbert, both at the University of Chicago, Illinois, created a simulation of ancient Earth. They found that if hydrogen made up a tenth of the atmosphere and nitrogen was present at double or triple today's level, ancient Earth's average temperature would have been 10 to 15 °C higher - above the freezing point of water (Science, doi.org/j47). The hydrogen could have come from volcanoes, they say, and could have persisted as microbes that consume the gas may have been rare due to a lack of nutrients.

It's a good model, says Chris McKay of the NASA Ames Research Center in Moffett Field, California, but he says strong evidence will be needed to show that the elements were once that abundant.

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